1. Field of the Invention
This invention relates to an aircraft aircrew life support apparatus and is particularly concerned with integrated breathing demand regulator and garment inflation pressure control systems.
2. Description of the Prior Art
The enhanced agility of modern high performance aircraft designs give such aircraft the ability to perform very highly accelerative manoeuvres both at low altitude and at high altitudes, e.g. in excess of 12000 meters (40000 ft). To take advantage of this agility an aircrew member flying the aircraft must be protected against G-induced loss of consciousness, known as G-loc, as well as the effect of increasing altitude. In this regard, unless otherwise specified, references to altitude are to be understood as references to the altitude equivalent to the pressure within an enclosure or cabin within which an aircrew member is situated and which is usually pressurised in relation to the external ambient pressure with the consequence that "cabin altitude" is related to but usually less than the actual altitude of the aircraft.
The partial pressure of oxygen in air decreases with increasing altitude (decreasing total pressure) so that the concentration of oxygen in breathing gas supplied to the aircraft aircrew member must be increased with increasing cabin altitude to maintain the oxygen partial pressure above the minimum value necessary for it to be able to diffuse through the lung tissue and pass to the haemoglobin or red corpuscles in the blood. If at aircraft operating altitudes above 12000 meters there is total or partial loss of cabin pressurisation or for other reasons the cabin altitude exceeds 12000 meters, the overall pressure of the breathing gas delivered to the aircrew member must be increased to a value above cabin ambient pressure so that the minimum critical oxygen pressure is maintained in the lungs, this being referred to as positive pressure breathing (PPB).
Positive pressure breathing at high altitude is aided by exerting pressure around the chest to assist the aircrew member in exhaling used gas from his lungs against the positive pressure in his breathing mask. To meet this requirement the aircrew member wears an inflatable counter-pressure garment ("jerkin") around his chest and back area which is inflated to the same pressure as the pressure in the breathing mask during positive pressure breathing, conveniently by being connected for inflation by breathing gas delivered to the breathing mask.
The effects on an aircrew member of high G-load experienced when performing high speed turning manoeuvres are well appreciated. The problem of G-loc is described in an article entitled "G-loc: taming the killer" by Mr. Mike Gaines which appeared at pages 27 to 30 of the Mar. 28, 1987 edition of "FLIGHT INTERNATIONAL".
To counter the effects of high G-load the aircrew member wears an inflatable G-protection trouser garment ("G-suit") which is inflated from a source of high pressure gas, such as engine bleed air, in response to signals from one or more accelerometers located in the aircraft for sensing accelerative forces. When inflated, the trouser garment restricts the flow of blood into the lower extremeties of the body where it tends to be forced under the action of the G-load to which the aircrew member is subjected.
It has been found that protection against G-loc is further enhanced by providing positive pressure breathing during periods when high G-loads are being experienced. The increase in breathing pressure causes an approximately equal increase in heart level blood pressure thereby increasing the flow of blood to the brain.
At altitudes which demand positive pressure breathing it is advantageous to inflate the trouser garment to a pressure approximately four times that of the pressure in the breathing mask even at times when aircraft flight maneouvres are not such as to give rise to high G-load. This inflation of the trouser garment counteracts the tendency for blood to be forced into the lower extremities of the body by the high pressure in the lungs, to reduce the circulation of blood from the heart to the brain. However, when both altitude and G-load conditions give rise to a requirement for positive pressure breathing, the trouser garment should be inflated to the higher one of the pressure requirements for protection against the prevailing G-load and exposure to high altitude.
It is common practice now to provide oxygen-enriched air as breathing gas for an aircrew member of a high performance aircraft from an on-board oxygen generating system (OBOGS) which includes molecular sieve beds comprising zeolite material suited to the retention of nitrogen whilst permitting oxygen to pass through the beds.
A problem with respect to demand valve operation in a breathing regulator suitable for accommodating the lower range of breathing gas pressure available from an OBOGS is overcome by a breathing regulator disclosed in EP-A-0 263 677 (Normalair-Garrett) which provides positive pressure breathing when the cabin altitude exceeds 12000 meters and, also, when high G-loads are being experienced. Above 12000 meters cabin altitude, an aneroid valve expands to increasingly restrict the flow of gas from a breathing-pressure control chamber so that pressure in this control chamber increases thereby increasing the pressure of the breathing gas at the regulator outlet to which both breathing mask and counter-pressure garment or jerkin are connected.
When the aircrew member is subjected to high G-loads, i.e. between 3.5 G and 9 G, a further valve regulating outflow from the breathing-pressure control chamber is signalled pneumatically by an anti-G valve to move towards increasingly restricting outflow of gas from the breathing-pressure control chamber so that pressure in that chamber increases to provide (increased) positive pressure breathing in the event that the cabin altitude is below that at which the same degree of positive pressure breathing would be provided. The anti-G valve is an electro-pneumo-mechanical device that controls a supply of inflation air to the G-suit in accordance with sensed G-loads and the signal to the further valve of the demand regulator is obtained by tapping the inflation air line from the anti-G valve to the G-suit.
Further disclosures of aircraft aircrew life support systems and apparatus which control inflation of a G-protection garment worn by the aircrew member and regulate delivery of breathing gas in accordance with the breathing demands of the aircrew member are to be found in U.S. patent application Ser. No. 4,230,097 (Intertechnique), U.S. patent application Ser. No. 4,638,791 (Boeing) and GB-A-2,051,417 (Intertechnique), this last disclosing a unitary or integrated breathing demand regulator and G-suit inflation control valve in which, however, the demand regulator and the control valve are functionally separate.
The prior art systems (other than EP-A-0,263,677) such as above discussed treat breathing gas and jerkin pressure requirements and G-suit inflation pressure requirements as separate functions to be provided by individual sub-systems integrated, functionally, only to the extent of sharing input data (such as anticipated and/or realised G-loads) output by a common source. There is therefore a need for greater functional integration of such sub-systems to provide better control and coordination of their respective functions and, especially to provide optimised responses to abrupt changes in aircraft flight conditions. Additionally there is the need, common in aircraft equipment, to minimise the weight and space requirements of equipment, especially cockpit equipment having close proximity to aircrew members. EP-A-0,263,677 partly meets the need for functional integration but has potential shortcomings in practice.